The present invention pertains to processes for pulping cellulosic materials, e.g., wood, bagasse and straw pulping.
In the manufacture of pulp from cellulosic materials for making paper, cardboard and related products the cellulosic fibers must be liberated from the cellulosic materials using liquid chemicals. The suspended solids of cellulosic fibers subsequently must be liberated and collected from the liquors.
The most common pulping processes employ cooking or digesting a cellulosic material, e.g. wood chips, to separate the cellulosic fibers from lignin and other materials in the wood chips. Side reactions of the pulping process produce high molecular weight organic by-products which include compounds of lignin, tar, pitch, saponified fatty acids and resins and other chemicals released from the wood chips.
During digesting processes, which can utilize a material, such as sulfate, caustic, sulfite, or alcohols, which is introduced into the digester along with the source of cellulosic fiber, with the digester raised to an elevated temperature and under pressure, the cellulosic fibers are released from the host material. Along with the fibers the high molecular weight organic by-products are released and become soluble in the liquid or pulping liquor. The high molecular weight organic by-products become dissolved or colloidal solids.
At the present time two commercial chemical processes are used to produce the cellulosic fiber material, i.e., Kraft or Sulfate Cooking and Acid or Sulfite Cooking. Another process that is the subject of promising laboratory work is Alcohol based Cooking (ASAM).
During and after the fiber producing material is cooked or digested and the fibers are released, the fibers are separated from the rest of the material and some or all of the spent cooking liquor can be used for further cooking of cellulosic material through circulating and recirculating the liquor. Dissolved solids in the spent liquor interfere in the initial cooking step or subsequent cooking steps. After the cooking process, the fiber is washed to remove residual cooking liquor and side reaction compounds. Further delignification can be obtained using oxygen and caustic which is commercially referred to as oxygen delignification within the washing sequence.
During the cleaning or washing processes during the manufacture of unbleached or bleached fibers, spent filtrate containing high molecular weight organic by-products can interfere with the physical and chemical processes needed to efficiently clean the fibers for the paper product. For example, a typical process of washing the fibers uses a cascading arrangement for the washing filtrate. Relatively clean water is introduced at the final washing stage and is cascaded back through several washing devices downstream to the first washing device. The washing equipment extracts the high molecular weight molecules which are carried on the fibers. The contaminated pulp, after washing yields a dirty filtrate which is sent to a preceding step in the washing process. Eventually, the high weight molecular organic by-products in the dirty filtrate interferes with efficient washing of the pulp. Also, residual high molecular weight material not removed from the fiber material interferes with a subsequent bleaching process.
Thus removal of the high weight molecular organic by-products will increase the efficiency of the cooking, washing, and bleaching processes involved in the production of pulp.
The present invention is a method for improving the overall process efficiency of a sulfite, Kraft or alcohol pulping process by removing dissolved high molecular weight organic by-products from any liquor or filtrate stream withdrawn from any process step by passing the liquor or filtrate stream through a filtration media that will trap the high molecular weight organic by-products and, returning the liquor or filtrate to any suitable point in the cooking, washing or bleaching process.
The liquor or filtrate freed of all or a portion of high molecular weight organic by-products will reduce the chemical or physical interference in a suitable process step that normally utilize the spent liquor or filtrate and thus increase the overall efficiency of the pulping process.
Thus in one aspect, the present invention is a method for improving the efficiency of the batch digesting step of a wood fiber pulping process comprising steps of; separating at least a portion of the liquor from wood pulp during or after the digesting step and passing the liquor through a filtration media to remove dissolved high molecular weight organic by-products from the liquor, and returning the liquor after removal of the high molecular weight organic by-products to any digester.
In another aspect, the present invention is a method for improving the overall efficiency of the batch digesting step in a Kraft pulping process comprising the steps of taking black liquor separated from wood pulp during or after the digesting step and passing the black liquor through a filtration media to remove dissolved high molecular weight organic by-products from the liquor, and returning the black liquor after removal of the high molecular weight organic by-products to a digester.
In still another aspect, the present invention is a method for improving the overall efficiency of the batch digesting steps of a sulfite pulping process comprising the steps of taking liquor separated from wood pulp during or after the digesting step and passing the liquor through a filtration media to remove dissolved high molecular weight organic by-products from the liquor, and returning the liquor after removal of the high molecular weight organic by-products to a digester.
In a further aspect, the present invention is a method for improving the efficiency of a wood pulping process using a continuous digester comprising a step of incorporating into a liquor recirculating or circulating system associated with the continuous digester a filtration media to remove high molecular weight organic by-products from the recirculating or circulating liquor.
A still further aspect the present invention is a method for improving the efficiency of a wood pulping process incorporating storage of spent liquor comprising the step of passing the liquor entering the storage facility to a filtration step to remove dissolved high molecular weight organic by-products from the liquor.
In still another aspect, the present invention is a method for improving the efficiency of a wood pulping process incorporating storage of one of spent liquor or filtrate, comprising the step of either prior to or upon withdrawing the liquor or filtrate from the storage facility passing the liquor or filtrate through a filtration media to remove dissolved high molecular weight organic by-products from the liquor or filtrate.
In yet a further aspect, the present invention is a method for improving the efficiency of a wood pulping process incorporating accumulation of hot liquor in pressurized vessels comprising the step of either prior to or upon withdrawing the liquor from the accumulation facility passing the liquor through a filtration media to remove dissolved high molecular weight organic by-products from the liquor.
Another aspect of the present invention is a method for improving the efficiency of a wood pulping process incorporating recycle of one of liquor or filtrate in the wood pulping process, comprising the step of passing liquor or filtrate from any process step through a filtration media to remove colloidal or dissolved high molecular weight organic by-products from the liquor or filtrate and recycling the liquor or filtrate to any appropriate step in the wood pulping process.
A further aspect of the present invention is a method for improving the efficiency of a wood pulping process including fiber washing comprising the steps of; passing collected washing liquid through a filtration media to remove colloidal and/or dissolved high molecular weight organic by-products from the washing liquid to produce a cleaned washing liquid, and using the cleaned washing liquid as a washing liquid or for pulp dilution.
A still further aspect of the present invention is a method for improving the efficiency of a wood pulping process that includes oxygen as a delignification step preceded by and followed by washing of pulp, comprising the steps of; separating washing fluid from the pulp after one of any washing step preceding or any washing step following the oxygen delignification step, passing the separated washing fluid from the pulp through a filtration media to remove colloidal and/or dissolved high molecular weight organic by-products from the washing fluid, to produce a cleaned washing fluid and using the cleaned washing fluid in any washing operation or to dilute the pulp prior to, after or during oxygen delignification.